The news that Microsoft was closing Age of Empires and Halo Wars producer Ensemble shocked quite a few people, but it also allowed the press to do some digging through their skeleton-ridden closets, and Gamasutra found some very interesting things.
According to Gamasutra, from 2006-2007 Ensemble was working on a MMO based in the Halo universe. The studio had even been hirring for an MMO project in early 2006. According to the website, the MMO was most likely internally cancelled in mid to late 2007 for reasons unknown.
As you can see from the images and concepts, the Halo MMO had a very similar layout to most MMOs out now including World of Warcarft. Talents at the bottom, health and "mana" at the top left among other similarities. One can begin to wonder, what if this had been completed? I guess we will never know.